Experiment 1: Deep tillage effects on waterhemp populations
In the fall of 2016 a field experiment was established to examine the effects of deep tillage on waterhemp populations the following season. Tillage treatments included: 1) fall moldboard plow followed by shallow spring tillage, 2) shallow spring tillage, and 3) no-tillage. Prior to fall tillage, 10 random soil cores (4-inch diameter) were taken to establish the baseline waterhemp seedbank. Soil cores were separated into two depths 0-2 and 2-7 inches deep. Additional soil cores (six subsamples per plot) at the same depths were taken following spring tillage. Half of the subsamples will be elutriated for waterhemp seed and counted, the other half will be grown out in the greenhouse and emerged waterhemp plants will be counted. Roundup Ready 2 Xtend soybeans were planted on May 18 in 30 inch rows. Within each tillage system a subset of herbicide treatments were established. These treatments included: 1) high waterhemp management (PRE fb. POST + residual), 2) standard waterhemp management (PRE fb. POST), and 3) no waterhemp management. Three 0.25 m2 permanent quadrats were established per plot for weekly waterhemp emergence counts and treatments were evaluated for weed control throughout the growing season. All treatments were replicated four times.
Results and observations:
• The initial soil seedbank of waterhemp from greenhouse grow outs from fall collected samples was 786 plants per 4-inch diameter core at the 0 to 2-inch depth and 141 plants at the 2 to 7-inch depth.
• There was not a difference between the total numbers of common waterhemp plants that emerged between the two tilled systems, moldboard plowed versus spring soil finish. However, over 2.5-times the number of waterhemp plants emerged in the no-till system.
• Total waterhemp emergence counts across different tillage systems were:
o No-till: 1606 plants per 0.25 m2
o Spring soil finished: 559 plants per 0.25 m2
o Moldboard plowed: 416 plants per 0.25 m2
• Waterhemp emergence in no-till plots peaked on May 26, averaging 498 plants per 0.25 m2. Waterhemp emergence in moldboard plowed and soil finished plots peaked one week later on May 30, averaging 428 plants and 201 plants per 0.25 m2, respectively.
• In the standard and high management plots, preemergence applications of Fierce at 3 oz/A provided good to excellent control of waterhemp for about six weeks.
• Postemergence follow up applications of XtendiMax (dicamba) and XtendiMax + Warrant (acetochlor) provided excellent control of waterhemp throughout the remainder of the season.
• Soybean yield was similar for the standard and high management treatments, and there was over a 10% reduction in soybean yield with the low management plot (no waterhemp control) combined over the three tillage systems.
• Tillage also affected soybean yield averaged across the three weed control programs. Yields were greater in the spring soil finish followed by moldboard plowed and no-till resulting in the lowest yield. This was likely due to the higher populations of waterhemp in the no-till low management plots.
• Initial results from our spring soil seedbank analysis, indicate that there is a large difference in the number of seeds in the 0-2” and 3-7” depths and that seedbank appears to be lower in the moldboard plowed area. We are currently analyzing the samples taken in the fall.
Experiment 2: Waterhemp control systems in Roundup Ready 2 Xtend and LibertyLink soybean
A field experiment was established to compare three different soybean trait systems for waterhemp control. Roundup Ready 2 Xtend ‘AG25X6’ and LibertyLink ‘DF9251LL’ soybean were planted in 30 inch rows on May 18. Following planting, PRE herbicide treatments of Valor at 2 oz/A were applied to selected treatments. Follow up POST treatments included: 1) XtendiMax + Roundup PowerMax, 2) XtendiMax + Roundup PowerMax + Warrant, and 3) XtendiMax + Roundup PowerMax + Warrant Ultra in the Roundup Ready 2 Xtend soybean. The Roundup Ready follow up treatments of 1) Roundup PowerMax, 2) Roundup PowerMax + Flexstar, and 3) Roundup PowerMax + Flexstar + Warrant were also applied to Roundup Ready 2 Xtend soybean. The follow up treatments in the Liberty Link soybean were: 1) Liberty, 2) Liberty + Warrant, and 3) Liberty + Warrant Ultra. Weed control was evaluated throughout the season and soybean yields were taken.
Results and observations:
• Soil-applied applications of Valor at 2 oz/A provided adequate control of waterhemp until 60 d after planting (DAP) when the POST herbicide applications were made.
• Soybean injury from POST applications only occurred when the active ingredient fomesafen (Flexstar or Warrant Ultra) was applied. Injury ranges from 7 to 12%, 15 d after treatment (DAT).
• Waterhemp control 15 DAT was greater than 90% with Liberty and XtendiMax treatments. Waterhemp control was between 80 to 88% when Flexstar was applied.
• Similar treatments held true with the 30 DAT evaluations.
• By the end of the season, all the LibertyLink and XtendiMax programs provided similar waterhemp control, with the Roundup Ready system being slightly lower.
• Within each system, yield was not different. With the exception of the Roundup only control plot (no waterhemp control) that reduced yield up to 34%. However, overall higher yields were observed with the LibertyLink soybean.
Experiment 3: Soil-applied (PRE) herbicides for waterhemp control in LibertyLink soybean
A field experiment was established to compare the residual activity of various soil-applied herbicides on waterhemp control. LibertyLink ‘DF9251LL’ soybean were planted in 30 inch rows on May 18. Following planting, 11 different PRE herbicide treatments were applied. Waterhemp control was monitored throughout the season. Liberty was applied when the majority of the PRE treatments needed a POST herbicide application for waterhemp control.
Results and observations:
• There was significant variation in the initial control and the length of residual activity for the different soil-applied herbicides.
• At the time of the POST Liberty application, 60 DAP, the treatments that provided the greatest waterhemp control were: Fierce, Fierce MTZ, Warrant Ultra, Zidua, Spartan + Metribuzin, and Valor + Metribuzin.
• After the POST Liberty application all treatments provided greater than 95% waterhemp control, with the exception of Metribuzin PRE applied alone at 8 oz/A followed by Liberty. At the time of the POST application control with Metribuzin was only 35%. At the time of harvest waterhemp control in this plot was only 70%.